Perhaps well known to others, but new to me: the U.S. Library of Congress' "Music for the Nation: American Sheet Music."
http://memory.loc.gov/ammem/mussmhtml/mussmhome.html
Appears to include many Catholic/sacred music offerings.
It's one of the better sources for late-19th c American Catholic church music. I haven't exactly been blown away by anything I've found there, but I haven't examined it exhaustively either.
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